Sources from this episode:

* Wikipedia: Glass (https://en.wikipedia.org/wiki/Glass)

* Science: Cathedral Glass Myth Shattered (http://www.sciencemag.org/news/1998/05/cathedral-glass-myth-shattered)

* Journal of the American Ceramic Society: Viscous flow of medieval cathedral glass (https://onlinelibrary.wiley.com/doi/full/10.1111/jace.15092)